title: Genetic and morphological divergences in the cosmopolitan deep-sea amphipod Eurythenes gryllus reveal a diverse abyss and a bipolar species.
abstract: Eurythenes gryllus is one of the most widespread amphipod species, occurring in every ocean with a depth range covering the bathyal, abyssal and hadal zones. Previous studies, however, indicated the existence of several genetically and morphologically divergent lineages, questioning the assumption of its cosmopolitan and eurybathic distribution. For the first time, its genetic diversity was explored at the global scale (Arctic, Atlantic, Pacific and Southern oceans) by analyzing nuclear (28S rDNA) and mitochondrial (COI, 16S rDNA) sequence data using various species delimitation methods in a phylogeographic context. Nine putative species-level clades were identified within E. gryllus. A clear distinction was observed between samples collected at bathyal versus abyssal depths, with a genetic break occurring around 3,000 m. Two bathyal and two abyssal lineages showed a widespread distribution, while five other abyssal lineages each seemed to be restricted to a single ocean basin. The observed higher diversity in the abyss compared to the bathyal zone stands in contrast to the depth-differentiation hypothesis. Our results indicate that, despite the more uniform environment of the abyss and its presumed lack of obvious isolating barriers, abyssal populations might be more likely to show population differentiation and undergo speciation events than previously assumed. Potential factors influencing species' origins and distributions, such as hydrostatic pressure, are discussed. In addition, morphological findings coincided with the molecular clades. Of all specimens available for examination, those of the bipolar bathyal clade seemed the most similar to the 'true' E. gryllus. We present the first molecular evidence for a bipolar distribution in a macro-benthic deep-sea organism.
authors.affiliation: Direction Natural Environment, Royal Belgian Institute of Natural Sciences, Brussels, Belgium ; Biodiversity Research Centre, Earth and Life Institute, Catholic University of Louvain, Louvain-la-Neuve, Belgium.
authors.forename: Charlotte
authors.initials: C
authors.lastname: Havermans
authors.forename: Gontran
authors.initials: G
authors.lastname: Sonet
authors.forename: Cédric
authors.initials: C
authors.lastname: d'Udekem d'Acoz
authors.forename: Zoltán T
authors.initials: ZT
authors.lastname: Nagy
authors.forename: Patrick
authors.initials: P
authors.lastname: Martin
authors.forename: Saskia
authors.initials: S
authors.lastname: Brix
authors.forename: Torben
authors.initials: T
authors.lastname: Riehl
authors.forename: Shobhit
authors.initials: S
authors.lastname: Agrawal
authors.forename: Christoph
authors.initials: C
authors.lastname: Held
mesh_terms: D033304:Amphipoda / Q000145:classification / Q000235:genetics*; D000818:Animals; D014644:Genetic Variation; D005843:Geography; D010802:Phylogeny; D013045:Species Specificity